Commit 0808d911 authored by Edward M. Reingold's avatar Edward M. Reingold

Initial revision

parent 5c19bda9
;;; cal-chinese.el --- calendar functions for the Chinese calendar.
;; Copyright (C) 1995 Free Software Foundation, Inc.
;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
;; Keywords: calendar
;; Human-Keywords: Chinese calendar, calendar, holidays, diary
;; This file is part of GNU Emacs.
;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING. If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
;;; Commentary:
;; This collection of functions implements the features of calendar.el,
;; diary.el, and holidays.el that deal with the Chinese calendar. It was
;; written by
;; Edward M. Reingold Department of Computer Science
;; (217) 333-6733 University of Illinois at Urbana-Champaign
;; reingold@cs.uiuc.edu 1304 West Springfield Avenue
;; Urbana, Illinois 61801
;;; Code:
(require 'lunar)
(defvar chinese-calendar-terrestrial-branch
["Zi" "Chou" "Yin" "Mao" "Chen" "Si" "Wu" "Wei" "Shen" "You" "Xu" "Hai"])
(defvar chinese-calendar-celestial-stem
["Jia" "Yi" "Bing" "Ding" "Wu" "Ji" "Geng" "Xin" "Ren" "Gui"])
(defvar chinese-calendar-time-zone
'(if (< year 1928)
(+ 465 (/ 40.0 60.0))
480)
"*Number of minutes difference between local standard time for Chinese
calendar and Coordinated Universal (Greenwich) Time. Default is for Beijing.
This is an expression in `year' since it changed at 1928-01-01 00:00:00 from
UT+7:45:40 to UT+8.")
(defvar chinese-calendar-location-name "Beijing"
"*Name of location used for calculation of Chinese calendar.")
(defvar chinese-calendar-daylight-time-offset 0
; The correct value is as follows, but I don't believe the Chinese calendrical
; authorities would use DST in determining astronomical events:
; 60
"*Number of minutes difference between daylight savings and standard time
for Chinese calendar. Default is for no daylight savings time.")
(defvar chinese-calendar-standard-time-zone-name
'(if (< year 1928)
"PMT"
"CST")
"*Abbreviated name of standard time zone used for Chinese calendar.")
(defvar chinese-calendar-daylight-time-zone-name "CDT"
"*Abbreviated name of daylight-savings time zone used for Chinese calendar.")
(defvar chinese-calendar-daylight-savings-starts nil
; The correct value is as follows, but I don't believe the Chinese calendrical
; authorities would use DST in determining astronomical events:
; '(cond ((< 1986 year) (calendar-nth-named-day 1 0 4 year 10))
; ((= 1986 year) '(5 4 1986))
; (t nil))
"*Sexp giving the date on which daylight savings time starts for Chinese
calendar. Default is for no daylight savings time. See documentation of
`calendar-daylight-savings-starts'.")
(defvar chinese-calendar-daylight-savings-ends nil
; The correct value is as follows, but I don't believe the Chinese calendrical
; authorities would use DST in determining astronomical events:
; '(if (<= 1986 year) (calendar-nth-named-day 1 0 9 year 11))
"*Sexp giving the date on which daylight savings time ends for Chinese
calendar. Default is for no daylight savings time. See documentation of
`calendar-daylight-savings-ends'.")
(defvar chinese-calendar-daylight-savings-starts-time 0
"*Number of minutes after midnight that daylight savings time starts for
Chinese calendar. Default is for no daylight savings time.")
(defvar chinese-calendar-daylight-savings-ends-time 0
"*Number of minutes after midnight that daylight savings time ends for
Chinese calendar. Default is for no daylight savings time.")
(defun chinese-zodiac-sign-on-or-after (d)
"Absolute date of first new Zodiac sign on or after absolute date d.
The Zodiac signs begin when the sun's longitude is a multiple of 30 degrees."
(let* ((year (extract-calendar-year
(calendar-gregorian-from-absolute
(floor (calendar-absolute-from-astro d)))))
(calendar-time-zone (eval chinese-calendar-time-zone))
(calendar-daylight-time-offset
chinese-calendar-daylight-time-offset)
(calendar-standard-time-zone-name
chinese-calendar-standard-time-zone-name)
(calendar-daylight-time-zone-name
chinese-calendar-daylight-time-zone-name)
(calendar-calendar-daylight-savings-starts
chinese-calendar-daylight-savings-starts)
(calendar-daylight-savings-ends
chinese-calendar-daylight-savings-ends)
(calendar-daylight-savings-starts-time
chinese-calendar-daylight-savings-starts-time)
(calendar-daylight-savings-ends-time
chinese-calendar-daylight-savings-ends-time))
(floor
(calendar-absolute-from-astro
(solar-date-next-longitude
(calendar-astro-from-absolute d)
30)))))
(defun chinese-new-moon-on-or-after (d)
"Absolute date of first new moon on or after absolute date d."
(let* ((year (extract-calendar-year
(calendar-gregorian-from-absolute d)))
(calendar-time-zone (eval chinese-calendar-time-zone))
(calendar-daylight-time-offset
chinese-calendar-daylight-time-offset)
(calendar-standard-time-zone-name
chinese-calendar-standard-time-zone-name)
(calendar-daylight-time-zone-name
chinese-calendar-daylight-time-zone-name)
(calendar-calendar-daylight-savings-starts
chinese-calendar-daylight-savings-starts)
(calendar-daylight-savings-ends
chinese-calendar-daylight-savings-ends)
(calendar-daylight-savings-starts-time
chinese-calendar-daylight-savings-starts-time)
(calendar-daylight-savings-ends-time
chinese-calendar-daylight-savings-ends-time))
(floor
(calendar-absolute-from-astro
(lunar-new-moon-on-or-after
(calendar-astro-from-absolute d))))))
(defun calendar-absolute-from-chinese (date)
"The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
The Gregorian date Sunday, December 31, 1 BC is imaginary."
(let* ((cycle (car date))
(year (car (cdr date)))
(month (car (cdr (cdr date))))
(day (car (cdr (cdr (cdr date)))))
(g-year (+ (* (1- cycle) 60);; years in prior cycles
(1- year);; prior years this cycle
-2636));; years before absolute date 0
(new-year (chinese-new-year g-year))
(current-month new-year)
(current-month-number 1)
(next-month (chinese-new-moon-on-or-after (1+ new-year)))
(next-sign (chinese-zodiac-sign-on-or-after
(1+ (chinese-zodiac-sign-on-or-after current-month))))
(had-leap-month nil))
(while (< current-month-number month)
;; current-month < next-month <= next-sign
(setq current-month next-month)
(setq next-month (chinese-new-moon-on-or-after (1+ current-month)))
(if (and (<= next-month next-sign) (not had-leap-month))
(progn;; leap month
(setq current-month-number (+ current-month-number 0.5))
(setq had-leap-month t))
(setq current-month-number (floor (1+ current-month-number)))
(setq next-sign (chinese-zodiac-sign-on-or-after (1+ next-sign)))))
(+ current-month (1- day))))
(defun calendar-chinese-from-absolute (date)
"Compute Chinese date (cycle year month day) corresponding to absolute DATE.
The absolute date is the number of days elapsed since the (imaginary)
Gregorian date Sunday, December 31, 1 BC."
(let* ((greg-date (calendar-gregorian-from-absolute date))
(greg-year (1- (extract-calendar-year greg-date)))
(greg-year
(+ greg-year
(calendar-sum y greg-year
(>= date (chinese-new-year (1+ y))) 1)) )
(chinese-year (+ greg-year 2697))
(cycle (/ (1- chinese-year) 60)) ;; previous cycles
(year (calendar-mod chinese-year 60));; years this cycle
(current-month (chinese-new-year greg-year))
(month 1)
(next-month (chinese-new-moon-on-or-after (1+ current-month)))
(next-sign (chinese-zodiac-sign-on-or-after
(1+ (chinese-zodiac-sign-on-or-after current-month))))
(had-leap-month nil))
(while (<= next-month date)
;; current-month < next-month <= next-sign
(setq current-month next-month)
(setq next-month (chinese-new-moon-on-or-after (1+ current-month)))
(if (and (<= next-month next-sign) (not had-leap-month))
(progn;; leap month
(setq month (+ month 0.5))
(setq had-leap-month t))
(setq month (floor (1+ month)))
(setq next-sign (chinese-zodiac-sign-on-or-after (1+ next-sign)))))
(list cycle year month (1+ (- date current-month)))))
(defun chinese-new-year (year)
"The absolute date of Chinese New Year in Gregorian YEAR."
(let* ((last-solstice (chinese-zodiac-sign-on-or-after
(calendar-absolute-from-gregorian
(list 12 15 (1- year)))))
(twelfth-new-moon;; twelfth month of previous year
(chinese-new-moon-on-or-after (1+ last-solstice)))
(thirteenth-new-moon;; maybe leap month, maybe New Year
(chinese-new-moon-on-or-after (1+ twelfth-new-moon)))
(fourteenth-new-moon;; maybe New Year, maybe second month
(chinese-new-moon-on-or-after (1+ thirteenth-new-moon)))
(next-solstice (chinese-zodiac-sign-on-or-after
(calendar-absolute-from-gregorian (list 12 15 year))))
(new-moons (+ 3 (calendar-sum m 0
(< (chinese-new-moon-on-or-after
(+ fourteenth-new-moon (* 29 m)))
next-solstice)
1))))
(if (and (= new-moons 14)
(< (chinese-zodiac-sign-on-or-after
(calendar-absolute-from-gregorian (list 2 15 year)))
thirteenth-new-moon)
(<= fourteenth-new-moon
(chinese-zodiac-sign-on-or-after
(calendar-absolute-from-gregorian (list 3 15 year)))))
fourteeth-new-moon
thirteenth-new-moon)))
(defun holiday-chinese-new-year ()
"Date of Chinese New Year."
(let ((m displayed-month)
(y displayed-year))
(increment-calendar-month m y 1)
(if (< m 5)
(let ((chinese-new-year
(calendar-gregorian-from-absolute
(chinese-new-year y))))
(if (calendar-date-is-visible-p chinese-new-year)
(list (list chinese-new-year
(format "Chinese New Year (%s-%s)"
(aref chinese-calendar-celestial-stem
(% (+ y 6) 10))
(aref chinese-calendar-terrestrial-branch
(% (+ y 8) 12))))))))))
(defun calendar-chinese-date-string (&optional date)
"String of Chinese date of Gregorian DATE.
Defaults to today's date if DATE is not given."
(let* ((a-date (calendar-absolute-from-gregorian
(or date (calendar-current-date))))
(c-date (calendar-chinese-from-absolute a-date))
(cycle (car c-date))
(year (car (cdr c-date)))
(month (car (cdr (cdr c-date))))
(day (car (cdr (cdr (cdr c-date)))))
(this-month (calendar-absolute-from-chinese
(list cycle year month 1)))
(next-month (calendar-absolute-from-chinese
(list cycle year (1+ month) 1)))
(month (floor month))
(m-cycle (% (+ (* year 5) month) 60)))
(format "Cycle %s, year %s (%s-%s), %smonth %s, day %s (%s-%s)"
cycle
year
(aref chinese-calendar-celestial-stem (% (+ year 9) 10))
(aref chinese-calendar-terrestrial-branch (% (+ year 11) 12))
(if (not (integerp month))
"second "
(if (< 30 (- next-month this-month))
"first "
""))
month
day
(aref chinese-calendar-celestial-stem (% (+ a-date 4) 10))
(aref chinese-calendar-terrestrial-branch (% (+ a-date 2) 12)))))
(defun calendar-print-chinese-date ()
"Show the Chinese date equivalents of date."
(interactive)
(message "Computing Chinese date...")
(message "Chinese date: %s"
(calendar-chinese-date-string (calendar-cursor-to-date t))))
(defun calendar-goto-chinese-date (date &optional noecho)
"Move cursor to Chinese date DATE.
Echo Chinese date unless NOECHO is t."
(interactive
(let* ((c (calendar-chinese-from-absolute
(calendar-absolute-from-gregorian
(calendar-current-date))))
(cycle (calendar-read
"Cycle number (>44): "
'(lambda (x) (> x 44))
(int-to-string (car c))))
(year (calendar-read
"Year in cycle (1..60): "
'(lambda (x) (and (<= 1 x) (<= x 60)))
(int-to-string (car (cdr c)))))
(month (read-minibuffer "Month: "))
(day (read-minibuffer "Day: ")))
(list (list cycle year month day))))
(calendar-goto-date (calendar-gregorian-from-absolute
(calendar-absolute-from-chinese date)))
(or noecho (calendar-print-chinese-date)))
(defun diary-chinese-date ()
"Chinese calendar equivalent of date diary entry."
(format "Chinese date: %s" (calendar-chinese-date-string date)))
(provide 'cal-chinese)
;;; cal-chinese ends here
;;; cal-coptic.el --- calendar functions for the Coptic/Ethiopic calendars.
;; Copyright (C) 1995 Free Software Foundation, Inc.
;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
;; Keywords: calendar
;; Human-Keywords: Coptic calendar, Ethiopic calendar, calendar, diary
;; This file is part of GNU Emacs.
;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING. If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
;;; Commentary:
;; This collection of functions implements the features of calendar.el and
;; diary.el that deal with the Coptic and Ethiopic calendars.
;; Comments, corrections, and improvements should be sent to
;; Edward M. Reingold Department of Computer Science
;; (217) 333-6733 University of Illinois at Urbana-Champaign
;; reingold@cs.uiuc.edu 1304 West Springfield Avenue
;; Urbana, Illinois 61801
;;; Code:
(require 'cal-julian)
(defvar coptic-calendar-month-name-array
["Tot" "Babe" "Hatur" "Kihak" "Tube" "Amshir" "Baramhat" "Barmuda"
"Bashnas" "Bauna" "Abib" "Misra" "Nisi"])
(defvar coptic-calendar-epoch (calendar-absolute-from-julian '(8 29 284))
"Absolute date of start of Coptic calendar = August 29, 284 A.D. (Julian).")
(defconst coptic-name "Coptic")
(defun coptic-calendar-leap-year-p (year)
"True if YEAR is a leap year on the Coptic calendar."
(zerop (mod (1+ year) 4)))
(defun coptic-calendar-last-day-of-month (month year)
"Return last day of MONTH, YEAR on the Coptic calendar.
The 13th month is not really a month, but the 5 (6 in leap years) day period of
Nisi (Kebus) at the end of the year."
(if (< month 13)
30
(if (coptic-calendar-leap-year-p year)
6
5)))
(defun calendar-absolute-from-coptic (date)
"Compute absolute date from Coptic date DATE.
The absolute date is the number of days elapsed since the (imaginary)
Gregorian date Sunday, December 31, 1 BC."
(let ((month (extract-calendar-month date))
(day (extract-calendar-day date))
(year (extract-calendar-year date)))
(+ (1- coptic-calendar-epoch);; Days before start of calendar
(* 365 (1- year)) ;; Days in prior years
(/ year 4) ;; Leap days in prior years
(* 30 (1- month)) ;; Days in prior months this year
day))) ;; Days so far this month
(defun calendar-coptic-from-absolute (date)
"Compute the Coptic equivalent for absolute date DATE.
The result is a list of the form (MONTH DAY YEAR).
The absolute date is the number of days elapsed since the imaginary
Gregorian date Sunday, December 31, 1 BC."
(if (< date coptic-calendar-epoch)
(list 0 0 0);; pre-Coptic date
(let* ((approx (/ (- date coptic-calendar-epoch)
366)) ;; Approximation from below.
(year ;; Search forward from the approximation.
(+ approx
(calendar-sum y approx
(>= date (calendar-absolute-from-coptic (list 1 1 (1+ y))))
1)))
(month ;; Search forward from Tot.
(1+ (calendar-sum m 1
(> date
(calendar-absolute-from-coptic
(list m
(coptic-calendar-last-day-of-month m year)
year)))
1)))
(day ;; Calculate the day by subtraction.
(- date
(1- (calendar-absolute-from-coptic (list month 1 year))))))
(list month day year))))
(defun calendar-coptic-date-string (&optional date)
"String of Coptic date of Gregorian DATE.
Returns the empty string if DATE is pre-Coptic calendar.
Defaults to today's date if DATE is not given."
(let* ((coptic-date (calendar-coptic-from-absolute
(calendar-absolute-from-gregorian
(or date (calendar-current-date)))))
(y (extract-calendar-year coptic-date))
(m (extract-calendar-month coptic-date)))
(if (< y 1)
""
(let ((monthname (aref coptic-calendar-month-name-array (1- m)))
(day (int-to-string (extract-calendar-day coptic-date)))
(dayname nil)
(month (int-to-string m))
(year (int-to-string y)))
(mapconcat 'eval calendar-date-display-form "")))))
(defun calendar-print-coptic-date ()
"Show the Coptic calendar equivalent of the selected date."
(interactive)
(let ((f (calendar-coptic-date-string (calendar-cursor-to-date t))))
(if (string-equal f "")
(message "Date is pre-%s calendar" coptic-name)
(message f))))
(defun calendar-goto-coptic-date (date &optional noecho)
"Move cursor to Coptic date DATE.
Echo Coptic date unless NOECHO is t."
(interactive (coptic-prompt-for-date))
(calendar-goto-date (calendar-gregorian-from-absolute
(calendar-absolute-from-coptic date)))
(or noecho (calendar-print-coptic-date)))
(defun coptic-prompt-for-date ()
"Ask for a Coptic date."
(let* ((today (calendar-current-date))
(year (calendar-read
(format "%s calendar year (>0): " coptic-name)
'(lambda (x) (> x 0))
(int-to-string
(extract-calendar-year
(calendar-coptic-from-absolute
(calendar-absolute-from-gregorian today))))))
(completion-ignore-case t)
(month (cdr (assoc
(capitalize
(completing-read
(format "%s calendar month name: " coptic-name)
(mapcar 'list
(append coptic-calendar-month-name-array nil))
nil t))
(calendar-make-alist coptic-calendar-month-name-array
1 'capitalize))))
(last (coptic-calendar-last-day-of-month month year))
(day (calendar-read
(format "%s calendar day (1-%d): " coptic-name last)
'(lambda (x) (and (< 0 x) (<= x last))))))
(list (list month day year))))
(defun diary-coptic-date ()
"Coptic calendar equivalent of date diary entry."
(let ((f (calendar-coptic-date-string (calendar-cursor-to-date t))))
(if (string-equal f "")
(format "Date is pre-%s calendar" coptic-name)
f)))
(defconst ethiopic-calendar-month-name-array
["Maskarram" "Tekemt" "Hadar" "Tahsas" "Tarr" "Yekatit" "Magawit" "Miaziah"
"Genbot" "Sanni" "Hamle" "Nas'hi" "Pagnem"])
(defconst ethiopic-calendar-epoch (calendar-absolute-from-julian '(8 29 7))
"Absolute date of start of Ethiopic calendar = August 29, 7 A.D. (Julian).")
(defconst ethiopic-name "Ethiopic")
(defun calendar-absolute-from-ethiopic (date)
"Compute absolute date from Ethiopic date DATE.
The absolute date is the number of days elapsed since the (imaginary)
Gregorian date Sunday, December 31, 1 BC."
(let ((coptic-calendar-epoch ethiopic-calendar-epoch))
(calendar-absolute-from-coptic date)))
(defun calendar-ethiopic-from-absolute (date)
"Compute the Ethiopic equivalent for absolute date DATE.
The result is a list of the form (MONTH DAY YEAR).
The absolute date is the number of days elapsed since the imaginary
Gregorian date Sunday, December 31, 1 BC."
(let ((coptic-calendar-epoch ethiopic-calendar-epoch))
(calendar-coptic-from-absolute date)))
(defun calendar-ethiopic-date-string (&optional date)
"String of Ethiopic date of Gregorian DATE.
Returns the empty string if DATE is pre-Ethiopic calendar.
Defaults to today's date if DATE is not given."
(let ((coptic-calendar-epoch ethiopic-calendar-epoch)
(coptic-name ethiopic-name)
(coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
(calendar-coptic-date-string date)))
(defun calendar-print-ethiopic-date ()
"Show the Ethiopic calendar equivalent of the selected date."
(interactive)
(let ((coptic-calendar-epoch ethiopic-calendar-epoch)
(coptic-name ethiopic-name)
(coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
(call-interactively 'calendar-print-coptic-date)))
(defun calendar-goto-ethiopic-date (date &optional noecho)
"Move cursor to Ethiopic date DATE.
Echo Ethiopic date unless NOECHO is t."
(interactive
(let ((coptic-calendar-epoch ethiopic-calendar-epoch)
(coptic-name ethiopic-name)
(coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
(coptic-prompt-for-date)))
(calendar-goto-date (calendar-gregorian-from-absolute
(calendar-absolute-from-ethiopic date)))
(or noecho (calendar-print-ethiopic-date)))
(defun diary-ethiopic-date ()
"Ethiopic calendar equivalent of date diary entry."
(let ((coptic-calendar-epoch ethiopic-calendar-epoch)
(coptic-name ethiopic-name)
(coptic-calendar-month-name-array ethiopic-calendar-month-name-array))
(diary-coptic-date)))
(provide 'cal-coptic)
;;; cal-coptic.el ends here
This diff is collapsed.
;;; cal-iso.el --- calendar functions for the ISO calendar.
;; Copyright (C) 1995 Free Software Foundation, Inc.
;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
;; Keywords: calendar
;; Human-Keywords: ISO calendar, calendar, diary
;; This file is part of GNU Emacs.
;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING. If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
;;; Commentary:
;; This collection of functions implements the features of calendar.el and
;; diary.el that deal with the ISO calendar.
;; Comments, corrections, and improvements should be sent to
;; Edward M. Reingold Department of Computer Science
;; (217) 333-6733 University of Illinois at Urbana-Champaign
;; reingold@cs.uiuc.edu 1304 West Springfield Avenue
;; Urbana, Illinois 61801
;;; Code:
(require 'calendar)
(defun calendar-absolute-from-iso (date)
"The number of days elapsed between the Gregorian date 12/31/1 BC and DATE.
The `ISO year' corresponds approximately to the Gregorian year, but
weeks start on Monday and end on Sunday. The first week of the ISO year is
the first such week in which at least 4 days are in a year. The ISO
commercial DATE has the form (week day year) in which week is in the range
1..52 and day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 =
Sunday). The Gregorian date Sunday, December 31, 1 BC is imaginary."
(let* ((week (extract-calendar-month date))
(day (extract-calendar-day date))
(year (extract-calendar-year date)))
(+ (calendar-dayname-on-or-before
1 (+ 3 (calendar-absolute-from-gregorian (list 1 1 year))))
(* 7 (1- week))
(if (= day 0) 6 (1- day)))))
(defun calendar-iso-from-absolute (date)
"Compute the `ISO commercial date' corresponding to the absolute DATE.
The ISO year corresponds approximately to the Gregorian year, but weeks
start on Monday and end on Sunday. The first week of the ISO year is the
first such week in which at least 4 days are in a year. The ISO commercial
date has the form (week day year) in which week is in the range 1..52 and
day is in the range 0..6 (1 = Monday, 2 = Tuesday, ..., 0 = Sunday). The
absolute date is the number of days elapsed since the (imaginary) Gregorian
date Sunday, December 31, 1 BC."
(let* ((approx (extract-calendar-year
(calendar-gregorian-from-absolute (- date 3))))
(year (+ approx
(calendar-sum y approx
(>= date (calendar-absolute-from-iso (list 1 1 (1+ y))))
1))))
(list
(1+ (/ (- date (calendar-absolute-from-iso (list 1 1 year))) 7))
(% date 7)
year)))
(defun calendar-iso-date-string (&optional date)
"String of ISO date of Gregorian DATE.
Defaults to today's date if DATE is not given."
(let* ((d (calendar-absolute-from-gregorian
(or date (calendar-current-date))))
(day (% d 7))